354 PACIFIC OCEAN. 
1. Cinder Cones. 
Cinder cones in the parts of the Pacific under examination, are of 
various heights, to two thousand feet. They have smooth and steep 
sides, and usually quite regular forms, though often more lengthened 
or of greater height on the leeward side, in consequence of the action 
of the winds on the ejected material. The angle of inclination is 
generally between thirty-five and forty degrees.* As has been often 
explained, these cones are the result of ejections of the melted lava to 
such heights that it falls in showers of dry cooled scoria, cinders, or 
ashes. The bursting of bubbles of vapour, rising in the viscid lava, 
is the cause of the ejection; and the compression the vapour under- 
goes, (the amount depending on the viscidity of the lava,) constitutes 
the projectile force, the strength of which is farther modified by the 
size of the vent or chimney (as compared with the size of the bubbles) 
through which the compressed air acts. The colour of these cones is 
black, or some dark shade like the lavas, unless subsequently altered 
to red by decomposition, which developes and changes the contained 
iron to red oxyd. 
The peak Tafua of Upolu is a fine example of these cones. Its 
. slopes scarcely vary from forty degrees. 
aa Assumption Island, one of the northern 
Ladrones, a correct outline of which 
is here given, is another instance, and 
one of special interest, as the simple 
cone stands alone in the ocean; the angle of inclination is the same 
as in 'Tafua. 
These cones generally rest on a base of lava; for they either follow 
or attend lava ejections. They are often of mixed character, as the 
lava cone is frequently finished off with a summit of cinders; cinder 
eruptions being usually the last effects of the subsiding fires. Mount 
Kea is a grand example of a mountain cone finishing its career as an 
eruptive volcano by the formation of a number of cinder cones at 
OUTLINE OF ASSUMPTION ISLAND. 
* Humboldt gives the angle as thirty-three to forty degrees. He states that the 
steepest possible angle, proceeding from loose cinders, is forty-two degrees, (Personal 
Narrative, Eng. Trans. i, 205, 206.) M. Leblanc (Bull. Soc. Geol. de France, xiv. 85, 
1843), after various observations in the Vosges and the Jura, lays down 35 degrees as 
the maximum slope ; 85° 16’ is the inclination of the diagonal of a cube. 
